Rotor generating



June 14, 1949. J. E. WHITFIELD ROTOR GENERATING 5 Sheets-Sheet 1 Filed Jan. 12, 1943 June 14, 1949. J. E. WHITFIELD ROTOR GENERAIING 5 Sheets-Sheet 2 Filed Jan. 12, 1943 INVENTOR.

fivm June 14, 1949. .1. E. WHITFIELD 3 ROTOR GENERATING Filed Jan. 12, 1945 5 Shee'ts-Shet 5 INVENTOR.

' BY I Zia ROTOR GENERATING 5 Sheets-Shet 4 Filed Jan. 12, 1943 June 14, 1949. J. E. WHITFIELD ROTOR GENERATING 5 Sheets-Sheet 5 Filed Jan. 12, 1943 Patented June 14, 1949 UN- l PATENT QEELGE .KROTOR'LGENERATING Joseph E5 Whitfield, Hamilton; Ohio Application January 12, 1943,'- Serial No; 472,132

1 This invention relates-generally togear cutting and more particularlyto themethodandapparatus for forming the working. facesof the helical gear-forms of the male and female rotors of a screw blower by generation with .a cutter that .has

relative reciprocating. movement with the blank and which- .intermeshes with the blank after the manner of amating gear and rotates. .intimed relation therewithduringthe generating oper- -ation.

The. screw blower. .-male .and. female. rotor membersto be formed have-complementary interme'shing helical.teeth;1the.curved sides of'the h'e'licalteeth of the male. rotor... being -described by thec'ontinuous crest edges. of. 'thelhelical teeth of the femalerotor an'dthe curved troughsof the female rotor teeth bein 'describedhythe continuous crest edgesofthe .helical teeth of the male rotor. 'Thefiill depth ofi'the tee'th ofthe male rotor are, preferablyifull or all addendum, 1

the pitch circle being less than the root diameter. The complementary teeth and troughs of the female rotor .are preferably rail or. all "deden'dum, the pitch circle beingigreater than the outside diameter of. the rotor. Again there isno clearance exceptthe normal'running clearance between the crests andtroughsof these rotor'teeth. .These -rotors are shown anddescri-bed in Letters Patent The principal object of 'thisdnvehtion is the provision of a-methodandapparatus forcutting teeth on matched-rotorswhich are fully complementary in that the teeth of "one are totally addendum and the teeth'of theothers are totally teeth onone'ofa matched pair ofrotors'by the point of a tool which represents thecrestedge of the complemental tooth-on the other rotor.

. Another object is the-'provision of -a-method and-.apparatus fon cutting 'teeth'on'rotors by "the erest edge of atool the' cross=sectional shape of which are complementary to the teeth.

Another object is the ':provision of a method and apparatus for'euttinghelical teeth on rotors by the use of atpointedtoblwhavingrelative rotary and longitudinal movement with respect to: and in timed?relatinn'rwithzthes rotor. rTl-le; limit of the 1 relative rotary? and::longitudinalr.movements for-one stroke of the-tool-is that'ofthe helix angle and the length of- .the --tooth-.an'd the feed is obttained. by rotatingthe-tool and the rotorin timed relation in steps after successive longitudinal relative movement.

.Another object ofthis vinventionis the cutting of complementary teeth on: rotors: by generation without form. cutters. Cutterweardoes not affect the tooth profile zbllt merely make.- itoversize which may be readily compensated for: by adjustment.

.--.Anot-her-=obj ect': is wthe; provision of apparatus .-for providing relative; rotary. movement between a cutting tool and a; gear blank.

Another 'object is the provision ofapparatus for providing relative -.longitudinal; movement between -.acutting tool and a gear; blank.

. Another object :is T the provision of :apparatus 5 for planingthe: surfaces :-.of rotor-teeth in which 5 1the:ourvedsurface is; produced bywa: tool having a stroke: of". relative :longitudinal :movement in? a curved path 'with-respect. to therotonwith-a feedingmovementzat the. end of each strokerproduced =by rotating the tool and -the rotorzin timed relation.

Other objects: and advantages appear: int the followingzdescriptionuand claims.

r slnltheiaecompanying :drawings arpractical em- --bocliment;illustrating the prineiplesxof thisrinven- :tion is shown' wherein:

T =Fig.v l isa sideview .ine elevationv of=a planer. and gear cutting planerattaehmentwith partsbroken away.

Fig-J2 is: anaendwiewfin; elevation taken onithe Fig. 3 isian .end'view in elevatiomtaken on the Fig. 4'15 a-sectional'vi'ew of ztheiplaner attachment.

"Fig; 5 -.is: an: end.view of. the' spline guide asform. of the-male rotorto'oth cutting tool.

.I'Fig. 11' isan-end view of the male and 'fem'ale rotors :ini mesh.

Fig; 12.. is a perspective view 1 of 'the niale and female'rotors :in mesh.

.rFig. 13..at:detailedasectional view ofa -modiform ofithatonl Fig. 14 is a, partial vievv 'of thegate-rotormemher illustrating a cutting tool for generating the left side of the trough.

Fig. is a partial view of the gate rotor member illustrating a cutting tool for generating the right side of the trough.

Fig. 16 is a diagrammatic view illustrating the relative positions of the cutting tool and the gate member at the beginning of the generating stroke, the rotor member being shown to illustrate the relative positions of the tool and the rotor member with respect to the gate as the latter is being generated.

Fig. 17 is a view similar to Fig. 16 showing the tool in the center of the generating stroke.

Fig. 18 is a view similar to Fig. 16 showing the tool nearing the end of the generating stroke.

Referring principally to Figs. 1, 2 and 3 of the drawing, it and H represent the columns of a planer which are rigidly connected to the machine bed, the upper surface of which is provided with ways for slidably supporting the planer table i2. A cross rail I3 is mounted to move vertically on ways formed on the front faces of the columns if: and l I. A pair of inwardly extending and oppositely disposed angle brackets M are fastened to the inner faces of the column of the machine and are positioned to just clear the table I2. A bridge member 15 is secured at its ends to the brackets Hi and spans the table 12 permitting the latter to traverse freely thereunder.

A housing I8 is carried by the bridge member l5 and is arranged to support and enclose the timing gear and feed mechanism of the planer attachment. The planer table l2 has four stands l3, l9 and 29 mounted in Spaced rela' 35 justed arcuately so that the splines in the liner tion, each carrying an axially aligned cap hearing 21. The cap bearing of the standards I! and is are arranged to rotatably support the shaft 22, the intermediate portion of which is provided with a helical splined section 23 which passes through the housing [6. If the teeth of the rotor members are straight through out their length the splined section should be straight. On the other hand if the teeth have a helical twist then the helix angle of the splined section 23 should be :2)

the same. The ends of the spline shaft 22 are provided with a thrust collar 24 and thrust rings 25 which abut against the end faces of the bearings 21 to prevent longitudinal movement of the shaft 22 and to transmit the axial thrust to the standards I! and i8 respectively.

The cap bearings of the standards I9 and 29 are arranged to rotatably support the shaft 26 which is also provided with thrust rings 21 that abut against the end faces of the bearings 21 to t prevent longitudinal movement of the shaft 2% and to transmit the axial thrust of said shaft to the standards 19 and 2B. Thus the axial thrust of each of the shafts 22 and 26 is independently transmitted to the table 12 and have no effect upon one another. The shafts: 22 and 2 6 are connected at their adjacent ends by the coupling 28 which rotatably drives the latter from the former and permits quick connection and disconnection of the shaft 26 from the assembly.

The rotor blanks 30, on which the helical gear teeth are out, are mounted on the shafts 2B. The blank shown in the drawings is that of a female rotor. These blanks may be cast oversize to the approximate shape of the rotor member and then placed on the machine for cutting. The shaft 25 may be removably mounted in the rotor blank 30 but it is preferable to cast the blank directly on the shaft 26, thus providing a permanent rotor and shaft structure.

The spline 23 on the shaft '22 mates with the splined nut 31 which is cradled in the universal joint assembly 32 carried on the end of the hollow shaft 33 that extends through and is journaled in the housing 16, as shown in Fig. 4. One end of the hollow shaft 33 is provided with a radially extending flange 34 having a pair of axially extending lugs 35 forming a yoke. Aligned holes are drilled transversely through the lugs 35 for rotatably receiving the stub shafts 38 which are clamped in sockets formed in the adjacent faces of the rings 37 by the bolts 38. Cap plates 59 are bolted to the outer surface of the lugs 35 for enclosing the stub shafts 36.

The rings 31 are provided with another set of aligned sockets disposed at ninety degrees from the lugs 35 in which the inwardly projecting stub shafts 43 are clamped by the bolts 38. The other end of the stub shafts 40 are rotatably received in aligned bearings on 0pposite sides of the casing M. The casing il has an inner lining 42 the bore of which has a spline formed complementary to the spline 23 on the shaft 22. The outer end of the casing ti is provided with a radially extending flange l3 and an enlarged bore 44 for receiving the flanged collar 45 which also has a matched splined inner lining 45. The flange of the collar A5 is secured to the flange :13 by the bolts ll. The bolt holes in the flange 43 are elongated to permit relative rotary adjustment of the collar 45 with respect to the casin ii. mien the splined liner sections 42 and t5 are worn from use the collar 45 may be adi2 engage one side of the spline on the shaft 22 while the splines in the liner 46 of the collar engage the other side. With this arrangement the wear may be taken up eliminating any lost motion or back lash, thus assuring accuracy in gear or rotor cutting.

Again the universal support for the splined nut 3i provides a flexible connection with continuous alignment in transmitting rotary power to or from the shaft 22 when the latter is reciprocated by the table 12 even through there may be slight variations relative to the hollow shaft 33.

The hollow shaft 33 extends beyond the other side of the housing l6 as shown in Fig. 4 to receive the worm wheel feed gear 48 which is driven by the worm 49 secured to one end of the shaft fill that is journaled in the bearing 5! fastened to the side of the casing l6. On the other side of the bearing 51 the shaft has a square toothed ratchet feed gear 52 secured thereto which is arranged to be stepped by the pawl or ratchet 53 pivotally supported on the upper short arm of the lever 54. The lever 54 is pivoted on the shaft 52 and is provided with a lower long arm having an adjustable block with an opening for slidably receiving the feed rod 55 which is pivotally supported at one end on the standard 56 secured to the table l2. Stops 57 are adjustably fastened to the rod 55 for engaging the long arm of the lever 54 at the end of a cutting stroke to rock the ratchet 53 and thus step the ratchet gear 52 and rotate the shaft 50, and thereby rotate the shaft 33 through the worm drive, which motion is transmitted to the shaft 22 by the splined nut 3 l.

The ratchet 53 may be thrown in either direction for effecting a rotary feed in either direction of the shaft 22. A hand crank 58 is secured tries 'a roller: 13 the axis of which' is horizontally 'idi'sposed. lBelow' theibearingill' the-Jbrack'et 12 isiprovided with a track M 1 whichsslidably cara central aperture for slidably receiving the trip moves away from the "wedge -block"which'- stays to the end :of theshaittll forinitiallyitposition- *ing the work-piece and the-tool on' for operating the feedmanually.

The cutting tool- 6B issecuredto one 'end of the shaft"- 6 lwhich is journaled in- "the tool head 62 slid'ably mounted to move vertically on-the planer cross rail tool slide. 1 The shaft 6 i 'extends back between the columns Hl'and ll to'the hous- 'ing I 6 where itis journaled -in-spacedbearing.

"The axis "of the shaft" 6 l 'is in' the same vertical plane as the" shafts 22; 26'" and 33-and is""provide 'with two"fiexib1e'couplings"53 between the tool'head 62 and; thelho'usin I t 'vvhicht'permit limited vertical and horizontal movement but are rigid-torsionally. A suitable fthrust' bearing 64' is provided onthe 'shaft'BI at the face of the tool head. 62- adjacent ithe-too1 6fl or assuming' the thrust. .forces-- when the tool is cutting.

.The upper end of the tool. head .62. is provided with-a stub shaft Bias-shown m Figs. '1. and- 3on whichthe intermc'ediate portion ofthe lever-66 is pivotally-supported. One end of the :leven-SB rests on the roller 61- rotatablymounted on the -b1ock=68 clamped to-the face of the planercross rail l3. -An operating-rod is pivotally connected to the other .end -of' the lever '66. The rod mpassesidown throughabearing H mounted :in .thebracket- 12- fast'ened: to the inner face" of the'column' II. The' lower end-of the rod carries the wedge b'lock 15. The wedgeblock has rod I6 which is supported at itsends by the "brackets "secured to the table' l2. "Tripfs'lides '18- may -be adjusta-bly secured to any 1 suitable position on the trip'rod 18 for engaging thewedge block 15 at'each'end'of the table-stroke' to'move the wedgingsurface into" or out of engagement with the roller 13.

When the table IZ-moVes the'work piece to the leitin Fig. 1 during a cutting stroke: the right trip 18 strikes the wedge" block; "moving it" along the" track 14 intoengagem'ent with the'roller'n at the. end of the cutting stroke to raise the tool out of cutting position. "The vertica lly slida'ble tool head 62 carries the" feed screw 69 which reciprocates through an opening in the crosshead tool slide. 'A collar 19 on the verticall' feed screw 69 strikes the top of the cros'shead tool slide and limits the downward movement or-the la-tterwand ther'ebyffixes 'the' lowermost positiont of'the tool T60. To regulate the depthof the out or ifthe 'tool wears and mustbe lowered further to. produce a ifinished cut, the. slidable tool head 82 maybe adjusted vertically by. meansof the vertical threaded feedscrewat the top. ofthe c ass railslide orbyadjustment of thecross rail.

JIEhe tool fillcutsfithe workpiece .during the movement of the table [2 to' the left iriFig. 1 and after the stroke has been completed the right trip '18 engages' the wedge'-block"l5 shoving it aIong the track' and into engagement with'the'riiller "13,causing it' to ride upthefinclinewwedging "surface andraise the tool head 62v lifting the tool out of its working position. The table His then reversedandmoves to'the right. "The trip then under the roller retaining the tool in itselevate'd position permitting the complete traverse'rofi the table l2 with the toolliftedout of cutting position. 'When the tool clears the w'orlg piececn its F movement to the "right the l =1ert-tri we again 7 engages the wedge: block I 75;:m0vingfiit outzfrom under the roller 1 3; thereby "permitting iitherrtool to-drop' to its-cutting position for thenext work ingstrokemf the planer. The table is then re- 5 versed a'nd the cutting stroke is repeated. This vertical rnoveme'nt of the tool head'is. permitted bythe fiex'iblecouplings' 63 in theshaft BI.

Referring again to Fig. 4 the hollow" shaft- 33 :and the shaft 6 I- are journaled in the stationary 10 housing lfi wlith Itheir'axes in the same vertic'al plane' 'and' are conne'cteduby two sets of timing gears which correlate the rotary movement-of the-tool 60 and the'rotor'blank 3H. As shown in Figs 11=and 12 the teeth of the rotor gear mem- 15 'bers have'a ratio'of two to four. The m'ale rotor 80, sometimes referred to as the -rotor',*has-"two teeth'or threads: and the female rotor 8 I :or' mate" has four teeth or threads. Thecre'st edges 82 of the male'rotor teeth generate tlie cor responding-sides of the female rotor troughs andthe :crest edges 8301 the female rotor teethgenerate the corresponding sidesof the male rotor-teeth. 'I huslthe plai-iing' tools-for cutting th'ese -teeth 'nes'clronlybepointed tools set equivalent: to the crest cdgesoi the respective rotor members.

=si-tezcrest edges of a female rotor thread 'for'cutting the' male rotor.

egain theimaleor female cutting toclmay' take -:theexacti'shape 'oi the rotor member as'shovvn hy theIfemale cutting tool 86 in -Figsfii and '9 whichhas'ei'ght cutting points 83. This typeof cutting tobl'is preferably used for producing the finishing cut on the rotor members.

Since the teeth of the rotor members have-a one to two ratio the timing gears in the housing 40 "461as='shown in Fig. 4 must also have 'thesarne 'ratio. Thus when'cutting a female-blocktfl by the cutter fifl; as shown in Fig. 1; the large timing gears-81 on the hollow shaft 33 in the housing H5 must be clamped by the clutch ringtt to the hub 89 that isrintegral with the shaft. The large'gear -81:meshes witlrthesrnall gear til which is fixed to-the shaiit fii and' the work piece is advanced :rhali' asm'uchas' the tool for feeding and for pro- -ducing the'cut along the helix chosen. "When to cutting the male rotor the converse is trueQ-the *worh piece ormale rotor blankmust be advanced twice as much as the tool. 'Thus the large gear r fikon-the shaft 6 i must be clamped-by the clutch ring 92i-to' the hub 93, which is keyed to the shaft sa -6|. "lhe"large' gear Si meshes with the small -gear fiat-keyed to the hollow shaft 33. Obviously Whenwne'set of timinggears are employed to einaintain theshafts 33' and 6! in timed-relation idui'ing the operation or the machine the other set isrpermitted 'to idle by loosening the correspending clutch ring. However the clutch ring flnot-beingz'employed to drive is permitted to drag, --thereby' creating sufiicient friction to eliminate r=lost motion or backlash of the gears to' prevent chattering between the woi'k'and' the cutter due toiany la'clr ot rigidity in mechanism.

In oper ating the gear cutting mechanism the idimensions and the pitch circles of the rotor 'menibers are flrst determined and the helixangle is s'electecl. Ihe spline'd section 23 of the shaft "22*is th'en' chosen toagree with the helix'angle "or thejgear teeth to be out. In some instances the helix angle may be different adjacent the ends of the rotor members than it is in the intermediate sectiongor' the teeth may have no helix angle, in which cases the splined section 23 must correspond. The work piece blank 3i) which is mounted on the shaft 26 is then placed in position on the standards l9 and 2c and the coupling 28 is connected. The trip slides it which control the raising and lowering of the tool are set relative to the length of the table stroke to permit their functioning before the ends of the stroke. The feed stops 51 are adjusted to produce the proper rotation of the tool and the work at the end of each cutting stroke to provide the feed desired for the next stroke of the planer table. The lowermost position of the tool to determine the depth of the cut is adjusted by the screw on the cross rail head and the proper timing gear clutch ring is clamped in operating position for coordinating the rotation of the tool relative to the work piece.

The machine is then placed in motion and the automatic trips which control the table traversing movements continue to automatically operate the gear cutting attachment.

With the shafts 33 and 5! locked together by the timing gears the tool is held stationary during the intermediate portion of the traverse of the planer table by the intermeshing of the feed worm and gear. The splined nut 3i is unable to turn and the reciprocal movement of the shaft 22 in passing through the stationary splined nut causes the work piece to be rotated in accordance with the helix of the splined shaft section 23. By operating the feed through the worm and gear the tool and the rotor blank are rotated in steps which generate the teeth on the blank. The gear teeth thus formed are accurate and true and are not dependent upon the shape or condition of the tool. Thus the feed generates the tooth profile and the spline generates the helix or length of the tooth.

Referring now to Fig. 13 it will be noted that the forward or tool end of the shaft 6|, which is journaled in the tool head 62, is provided with an outwardly flaring frusto conical surface 96 which mates with the complementary frusto conical surface in the fore part of the bearing.

The angle of taper of these mating conical bearing surfaces is selected to permit the cutting forces on the tool to force these surfaces into tight engagement and lock the shaft against movement to avoid tool chattering but the conical angle must be sufficiently large that the cutting force will not cause the mating frusto conical surfaces to freeze making it difficult for the shaft to be released at the end of the cutting stroke. The cutting point of the tool 50 being spaced from the shaft 6i aids in multiplying the reactive forces which jump the tool forward as it leaves the cutting surface. If the conical angle is properly chosen in accordance with a given cutting force the reactive forward kick of the cutting tool at the end of the cutting stroke aids in freezing the mating frusto conical surfaces. A suificient clearance is allowed between the locking rings 98 and the tool head 62 to permit the shaft 6! to move forward suficiently causing disengagement between the mating frusto conical surfaces. When there is no pressure on the cutting tool 68 the mating frusto conical surfaces permit the shaft 8! to freely revolve, but when the tool strikes the work the shaft iii is automatically locked. This tool shaft locking structure is an important object of this invention and its advantages enhance the operation of the generating machine.

In some rotor gear forms the cutting points 82 on the male cutting tool 60, for cutting the female gate rotor member, may be formed integral with the tool as described hereinbefore with reference to Figs. 6 and 7. However the deep arcuately shaped trough with the helical twist requires differently shaped cutting tools for forming the opposite sides of the troughs. The cutting points 82 and the adjacent parts of the tool may be shaped to conform with these requirements but it is considerably more economical to provide detachable cutting points or bits as illustrated at 99 and Hill in Figs. 1, 3 and 13 which are clamped to the ends of the cutting tool 60 by the plates Hill. Referring specifically to Fig. 3, the bit 99 is arranged to cut the adjacent side of the gate trough and the bit I00 cuts its adjacent side of the same trough, and both bits cut the bottom of the trough. During the transfer of the cutting tool 60 from one side of the gate trough to the other the bite of the cutting bit 99 gradually diminishes while the bit it! starts to cut. The bite of the bit I99 thus increases concurrently while the bite of the bit 99 decreases until all of the cutting load is transferred from one bit to the other as the cutting tool 68 is moved in successive steps from one side of the gate trough, across the bottom to the other by the operation of the automatic feed through the ratchet feed gear 52. Three positions of the cutting tool 68 and the gate rotor are illustrated in Figs. 16, 17 and 13. In Fig. 16 the bit 99 is cutting the left side of the trough and the bit 299 is clear of the work. Fig. 17 shows the center of the transition period where both bits 99 and Hill are cutting with the same bite. However during the next step of the feed in moving the tool to the right side of the gate rotor trough increases the bite of the bit I09 and concurrently decreases the bite of the bit 99. Fig. 18 shows the bit I cutting the right side of the trough and the bit 99 is clear of the work.

The profile of the bits 99 and 190 are rounded as shown in Figs. 16, 17 and 18, and the cutting points 82 on these bits are set in the tool 69 so that they are the same distance apart as the edges of the rotor thread when measured at right angles to the axis of the rotor. Thus the cutting points 82 when mounted on the tool 5!) correspond to the crest edges of the rotor member and thereby generate the trough of the gate when the work is reciprocated.

The rotor and gate members are ordinarily cast over size and the generating machine finishes them to size.

The heel 102 of the bit 99 as shown in Fig, 15 is substantially at right angles to the face of the bit, being backed off only slightly, whereas the rake of the heel 103 of the bit IE9 is considerable. By backing oil? the heels of the bits in this manner there is no tendency for them to ride the work and prevent the tip from cutting. The tool 100 does not provide the proper cutting action for the other side of the trough as it drags over the metal and the converse is true of the other tool bit 99. It is also necessary to provide a side rake on these bits because of the helix angle of the rotors. It is also necessary to properly form the cutting point as shown to provide clearance for the chips.

By using separate bits each may be properly ground to suit the particular work piece to be finished.

I claim:

1. In a gear box for use with a machine for generating complementary rotors, the combination, of a housing, parallel shafts journaled in the housing, one of said shafts being hollow and arranged to carry a splined nut, a. splined shaft mounted in the nut and arranged to rotate a rotor blank, the other of said first mentioned shafts arranged to rotate a tool, means for rotating one of said shafts, and a pair of intermeshed timing gears connecting said shafts for maintaining them in correlated timed relation.

2. The structure of claim 1 in which the means for rotating one of said shafts prevents said shafts from turning other than by said means.

3. In a machine tool the combination of a'tool supporting head, a shaft journaled in said head, a tool carried on one end of the shaft, a cutting bit carried by the tool, a support for carrying a work piece, means for producing relative reciprocating movement between the tool and the support for machining a work piece, a frusto conical section on the tool end of said shaft, a mating frusto conical surface in the tool head for receiving the frusto conical section of the shaft to lock the tool by the cutting forces when the cutting bit engages the work piece during a cutting stroke.

4. The structure of claim 3 wherein they angle of the mating frusto conical surfaces is selected to provide disengagement between the mating frusto conical surfaces upon sudden release of the cutting pressure when the bit leaves the work piece at the end of the cutting stroke.

5. The structure of claim 3 which also includes means for rotating the shaft and tool when the frusto conical surfaces are not locked in engagement by the cutting forces between the bit and the work piece.

6. The structure of claim 3 which also includes a. collar for limiting the relative longitudinal movement of the mating conical surfaces away from one another.

6 JOSEPH E. WHITFIIEID.

REFERENCES CITED The following referenlces are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 259,043 Reynolds June 6, 1882 497,997 Wilkin May 23, 1893 579,708 Fellows Mar. 30, 1897 1,103,851 Sykes July 14, 1914 1,388,853 Davis Aug. 30, 1921 1,415,341 Hanson May 9, 1922 1,423,266 Sears July 18, 1922 1,474,500 Wingqvist Nov. 20, 1923 1,669,919 Trbojevich May 15, 1928 1,680,258 Schurr Aug. 7, 1928 1,798,059 Bilgram et a1. Mar. 24, 1931 1,833,993 Hill Dec. 1, 1931 1,966,530 Symons July 17, 1934 1,997,228 Nichols Apr. 9, 1935 2,116,290 Spicer May 3, 1938 2,231,117 Greiner Feb. 11, 1941 FOREIGN PATENTS Number Country Date 216,671 Germany Nov. 30, 1909 

